1. Field of the Invention
The present invention relates to optical communication equipment.
2. Description of the Related Art
An optical communication system such as, for example, a Dense Wavelength Division Multiplexed (DWDM) communication system, may employ tunable lasers to generate optical signals corresponding to different communication channels of that system. A representative DWDM system may be configured to have 32 equally-spaced DWDM communication channels, e.g., occupying a wavelength range of about 25 nm from 1528 nm to 1553 nm. Each tunable laser in such a system would typically be capable of generating optical signals for each of the communication channels.
FIG. 1 schematically shows a representative tunable semiconductor laser 100 that can be deployed in a DWDM system. Laser 100 is a distributed Bragg-reflector (DBR) laser such as, for example, a sampled grating DBR (SG-DBR) laser or a grating-assisted co-directional coupler with rear-sampled reflector (GCSR) laser. Laser 100 is controlled by four control signals (typically currents) labeled G, C, R, and P in FIG. 1, each control signal applied to a different section of laser 100. Signal G (gain) controls the output power of laser 100, and signals C (coupler), R (reflector), and P (phase) control the output wavelength of laser 100. By appropriately changing signals C, R, and P, laser 100 can be tuned from one wavelength to another on a nanosecond time scale.
One problem with laser 100 is that, due to slight variations in the manufacturing process, different lasers 100 typically requires different combinations of control-signal amplitudes to generate light at a particular wavelength. In addition, for each wavelength, the combination of control-signal amplitudes will change over time due to the effects of aging. Laser control circuits (e.g., for supplying signals C, R, and P) that can efficiently accommodate device-to-device and/or aging variations are not currently available.
Another problem with laser 100 is that laser control circuits that are available support much slower switching (tuning) speeds than those physically sustainable by laser 100. For example, a widely tunable laser transmitter model 4245 available from Agility Communications, Inc., of Santa Barbara, Calif., can be switched between two different DWDM channels in as much time as about 10 ms.